The semi-colons in Javascript debate is probably decades old. (Which is quite a lot considering that the language itself is less that 2 decades old)
Most javascript developers are taught to always use semi-colons. The ASI in javascript is strange and inconsistent, and can surprise in weird ways.

So, let’s just not rely on stupid ASI and always write semi-colons. It sounds good enough. It sounds logical. Also, Douglas Crockford said it, so it’s probably the right thing to do.

But, the more time I spend writing semi-colons, the more I realize that you can’t really avoid the ASI by putting semi-colons. I agree that the JS ASI is weird and strange. But it is not inconsistent. It has a certain set of rules that work in every browser.

To use a saying made famous by Nokia, using semi-colons to avoid the ASI is like pissing in your pants to keep yourself warm. You see using semi-colons makes you forget about the ASI. You start assuming that your code will just work because you are putting semi-colons. But this is where we all go wrong.

The ASI surprises in two ways:

It doesn’t put a semi-colon when you expect it to.

In all the research I have done, this only happens when a line begins with an open paren, or bracket:

You see if the ASI can make sense of your code without putting a semi-colon it will do that. This is exactly why everyone puts semi-colons everywhere. For these two edge-cases. Really.

But here’s the flip side. Here is an example of when the ASI puts a semi-colon when you don’t expect it to:

return
someValue - 2
// becomes
// return;
// someValue - 2;

Well in the first case, if we put semi-colons everywhere, our problem would be solved. But about this case?

return
someValue - 2;

You know what gets returned? NOTHING. Yes. putting semi-colons manually everywhere doesn’t save you from the ASI putting in more semi-colons, when you don’t want them.

This is exactly why I think we should all stop writing semi-colons everywhere. We fool ourselves into thinking that if use semi-colons, we won’t have to worry about the ASI, but that couldn’t be further from the truth. The fact is that you still need to understand how the ASI works. You need to understand that if you purposely don’t put a semi-colon to make something work, Javascript won’t necessarily respect your expectations.

And if we do have to understand how the ASI works we might as well save ourselves some typing by not putting semi-colons when we don’t need them.

Further, and this is why I like the NPM style guide, if we don’t write semi-colons when we don’t need them, we can write more readable code that helps new developers really understand the ASI.

Take this for example:

// Works but doesn't teach you anything about the ASI.
var a = b;
[1].length;
var a = b;
(new Date()).valueOf();
// Works, but also teaches you about the ASI
var a = b
;[1].length
var a = b
;(new Date()).valueOf()

By using semi-colons before open parens and brackets, we write code that will teach new developers the nuances of the ASI. It will force them to understand the ASI, and protect themselves from unfortunate surprises. If we keep writing semi-colons everywhere, we are more likely to forget that the ASI even exists and run into surprises.

The world at large still likes to write the Crockford prescribed way of javascript, and it’s not going to be easy, but I think it’s time to try the NPM styles guide.